This invention relates generally to electric drive systems, and more particularly to drive systems using multi-phase alternating current (AC) motors.
One conventional electric drive application is the propulsion drive of underwater vehicles and the like. In such application, it is desirable to provide quiet propulsion at high power densities. It has further been conventional in electric propulsion systems to use direct current motors as the means for driving the propeller.
Because of increasing power requirements, it has been found desirable to use alternating current motors in lieu of the limited capacity direct current motors previously used. One proposed application of such alternating current motor drives uses a three-phase squirrel-cage induction motor excited by a three-phase half-bridge inverter connected to the battery power supply. It has been further proposed that such motors use a conventional three-phase winding configuration and that the inverters be arranged to provide three-phase, variable-frequency, variable-amplitude alternating current power to the motors. It is further proposed that amplitude control may be achieved by means of pulse width modulators.
In the proposed controls for use with such three-phase drive motors, power transistors are used as switching devices to reduce the weight of the propulsion system by eliminating the need for forced commutation circuitry and hardware. However, such available transistor devices are limited in the power switching capabilities and have been found to be inadequate to provide the desired propulsion system power levels at the present time. As a result, in the proposed systems, paralleling of transistors is necessitated to provide sufficient inverter power rating. Such paralleling of power transistors causes a number of serious technical problems. Because of the exacting weight and space requirements in electric propulsion vehicles, the use of additional equipment to prevent unequal current sharing among the parallel transistor devices has been a serious problem.
Another problem encountered in the three-phase motor drives used heretofore has been the production of substantial harmonic currents which cause pulsating torque components in the motor rotor which, in the prior art devices, may have amplitudes which are a substantial fraction of the average torque output of the system. Such harmonic currents result from the inverter delivering nonsinusoidal excitation waveforms to the motor. Thus, the three-phase half-bridge inverters conventionally employed deliver quasi-square wave (preferred to hereinafter as "six-step") voltage waveforms under the full speed and full power conditions. The pulsating torque developed by the harmonic currents is conventionally delivered directly to the speed reducing gear means in the drive system, which adversely affects the gear life and increases system noise due to gear chatter. The excitation harmonic currents further contribute to a substantial increase in the rotor losses in the drive motor, reducing the capability of the drive systems and presenting heat removal problems from the rotor assembly, which is desirably designed to be as compact as possible.
The problem of motor rotor torque pulsation has been treated by Rosenberry in U.S. Pat. No. 3,611,085 by using a plurality of electrically separate polyphase motor windings supplied by a plurality of inverters. Harmonic phase current content is directly related to the peak value of motor phase current. The limiting inverter switching device operating constraint is the peak value of the motor phase current. Inverter-driven motor configurations of the type described by Rosenberry will successfully treat torque pulsation but do not inherently suppress the 5th and 7th harmonic phase currents, and therefore, do not inherently limit peak motor phase current.
The suppression of motor phase current time harmonics, principally the 5th and 7th orders, was the goal of Meier (U.S. Pat. No. 3,792,286) and Hamilton (U.S. Pat. No. 4,220,881). The use of inductive reactors external to the motor, as described by Meier, would be extremely detrimental to propulsion system performance due to the large increase in drive train weight attributable to the reactors. This approach to harmonic suppression is, therefore, a very unattractive technical option. Hamilton has suggested a multiphase winding arrangement which provides a degree of harmonic suppression which is greater than prior practice, as described by the inverter-driven motors of the type employed in the practice of Rosenberry. The winding arrangement chosen was well suited for the particular number of phases discussed, but is not the best possible for any arbitrary number of phases.
Therefore, it is an object of the present invention to provide an electric drive system having improved reliability, efficiency, smooth torque production, and a high power to weight ratio.
It is another object of the invention to provide an electric drive system using an AC motor which provides optimal suppression of stator phase current harmonics in drive systems which do not employ inductive reactors external to the motor.
It is yet another object of the invention to provide an electric drive system which eliminates the need for paralleling of inverter switching devices.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.